<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<style>
    *{
        margin: 0;
        padding: 0;
        box-sizing: border-box;
        border: none;
    }
    body{
        height: 100vh;
    }
    .box{
        height: 100%;
        width: 100%;
        border: 1px solid darkgrey;

    }
    .box div{
        height: 33.3%;
        border: 1px solid yellow;
    }
</style>

<body>
    <div class="box">
        <div></div>
        <div></div>
        <div></div>
    </div>
</body>
</html>